Plants produce a wide diversity of secondary metabolites, many of which are volatile compounds emitted by leaves, flowers, roots and fruits, with different functions among which it has been found they act as signaling molecules in the interactions between plants or among distal areas from the same plant, as defense against pests and pathogens, as attractants for predators of herbivorous insects on leaves and roots, as attractants for insect pollinators in flowers, or as attractants for seed dispersal in fruits (Gershenzon and Dudareva, 2007). In addition, volatile compounds emitted by flowers are significant contributors to plant reproductive success and survival in natural ecosystems (Kessler et al., 2008). Finally, the aroma of plants and, more specifically, of their fruits, have significantly contributed to the selection of the best genotypes and their use by humans for nutritional, medicinal and industrial purposes (Goff and Klee, 2006).
In recent times, significant progress in understanding the biosynthetic pathways, in cloning important regulatory genes, in the purification of the enzymes and in the discovery of the regulatory mechanisms that lead to the formation of these volatile compounds and their emission by the different tissues or organs from plants have been achieved. Consequently, it has been proposed to use the knowledge obtained to improve plants through genetic engineering mainly with agronomic and nutritional purposes.
Citrus is the most economically important fruit tree crop in the world, with a production that exceeded 105 million tons in 2008 in an area of over 7.6 million hectares (FAO, 2009). They are grown in more than 130 countries in tropical and subtropical areas (up to 40 degrees latitude on either side of equator) where there are favorable soil and climatic conditions. The largest producers are Brazil, USA, China, Spain and Mexico, representing about 55% of world citriculture.
Citrus fruits are affected by important pests and diseases caused by nematodes, fungi, oomycetes, bacteria, spiroplasmas, phytoplasmas, viruses, viroids and diseases of unknown etiology. Some of these diseases affect most of the citrus cultivation areas, such as those caused by the oomycete Phytophthora sp. or by the Citrus Tristeza Virus (CTV), which prevent the use of certain excellent rootstocks and additionally restrict production and fruit quality in some countries. Others such as citrus canker, caused by Xanthomonas citri subsp. citri, that affects most important varieties, is widely expanded and now constitutes a serious threat to citrus in Florida and most South American countries. Other diseases are located in specific geographic areas, such as those produced by the bacterium Xylella fastidiosa in Sao Paulo (Brazil). Finally, there are diseases that have been locally important and in recent times have spread to other citrus-growing areas, such as Huanglongbing, caused by the bacterium Candidatus Liberibacter spp., affecting all varieties. This bacterium has prevented the development of a citrus industry in Southeast Asian countries and is currently destroying millions of trees in Florida and Brazil. In the case of the three above-mentioned bacteria, there are no effective means of control. There are also very important diseases affecting the post-harvesting of citrus fruits, such as those produced by fungi of the genus Penicillium. 
Regarding pests, there are some that directly affect the green parts of the tree and/or the fruit, as the Mediterranean fruit fly (Ceratitis capitata) and the California red scale (Aonidiella aurantii), and those that are vectors of diseases such as the psyllid Diaphorina citri, transmitting the bacteria Candidatus Liberibacter spp., or the aphid Toxoptera citricidus, very efficient vector of CTV. Although aggressive chemicals are currently being used to control citrus pests, they do not suppose a lasting, economically and/or environmentally sustainable solution in a medium term.
Given these serious threats to the citrus industry worldwide, it is a priority to search for alternative solutions, such as those based on genetic improvement. Despite efforts in classical breeding programs over more than a century, the current citrus industry is based on a small group of high-quality varieties that are grafted on a not too wide range of rootstocks. The great majority of these genotypes have been obtained randomly, i.e. they have been got from selection of spontaneous mutations detected in the field by farmers or from chance seedlings generated by germination in a fortuitous way. In addition, breeding programs are severely limited by the complex reproductive biology of citrus. In this context, genetic modification through transgenesis offers huge potential for improvement because it allows introducing unique traits in elite genotypes without altering their genetic background. Although social controversy exists about the use of this technology for plant breeding, we believe that the use of transgenes from the own citrus genome that one wish to modify would overcome the reluctance of certain sectors and especially if this strategy proved an advantage in environmental terms over the traditional ways used to control pests and pathogens.
In the last decade, a series of fundamental works on the role of plant volatiles as repellents of pests and attractants of predators of herbivores has been published (Aharoni et al., 2003, Arimura et al., 2000, De Moraes et al., 2001). This led to think that it was possible to modulate plant volatile emission through metabolic engineering for improving the response of plant defense against pests. Thus, overexpression of the gene precursor of a linalool/nerolidol synthase from strawberry in transgenic Arabidopsis led to accumulation of high levels of linalool and consequently to the induction of repellency against aphids (Aharoni et al., 2003). Overexpression of this transgene in Arabidopsis, but this time directed to mitochondria, led to the accumulation of nerolidol and a derived homoterpene, (E)-DMNT, which made plants attractive to insect carnivore predators, natural enemies of pest mites (Kappers et al., 2005). Along these lines, overexpression of the gene precursor of a sesquiterpene synthase, TPS10, in transgenic Arabidopsis plants made them appealing to parasitic bees of insect pests due to the emission of high levels of sesquiterpenes which are normally released when larvae of these bees chew the leaves (Schnee et al., 2006). More recently, overexpression of the gene precursor of a trans-caryophyllene synthase from oregano in transgenic corn made the plant roots attractants of nematodes that protect them from the attack of beetles pest (Degenhardt et al., 2009). Transgenic overexpression of a precursor gene of a pachulol synthase in tobacco together with a farnesyl diphosphate synthase, precursor of sesquiterpenes, led to a high accumulation of pachulol and 13 others sesquiterpenes which made plants highly resistant to larvae of insect pests (Wu et al., 2006).
The role of different terpenoid compounds to confer resistance to pathogens is well documented, particularly in forestry, but the overexpression of precursors of these genes as a biotechnology strategy of plant protection has not been reported so far (Trapp and Croteau, 2001).
In summary, all these studies suggest that the use of metabolic engineering to achieve resistance against biotic agents represents an alternative technology to the use of expensive fungicide products, bactericides, highly toxic pesticides and that its use would result in an increase in product quality.
Moreover, volatile compounds are important determinants of the perception of aroma and taste of the fruits by humans (Goff and Klee, 2006). Classical plant breeding has been concerned about maximizing attributes such as productivity or vigor, in detriment of other traits as the aroma, and this has led to a gradual losing of flavor and aroma in new varieties of many fruits. Furthermore, it has been proposed that some of the determinant compounds of such traits in fruits are beneficial to health (Bisignano and Saija, 2002). Today, the aroma is considered a quality attribute that should rejoin the new fruit varieties. Again, metabolic engineering is shown as a necessary technology for that purpose. Furthermore, it could allow the production of new combinations of scents in plants with industrial interest for food, perfumery, cosmetics, cleaning, etc.